Advanced split-illumination electron holography without Fresnel fringes

doi:10.1016/j.ultramic.2013.11.002

Ultramicroscopy

Volume 137, February 2014, Pages 7-11

https://doi.org/10.1016/j.ultramic.2013.11.002 Get rights and content

Highlights

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Advanced split-illumination electron holography without Fresnel fringes is developed.
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Two biprisms are installed in illuminating system of microscope.
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High-precision holographic observations of an area locating far from the sample edge become possible.

Abstract

Advanced split-illumination electron holography was developed by employing two biprisms in the illuminating system to split an electron wave into two coherent waves and two biprisms in the imaging system to overlap them. A focused image of an upper condenser-biprism filament was formed on the sample plane, and all other filaments were placed in its shadow. This developed system makes it possible to obtain precise reconstructed object waves without modulations due to Fresnel fringes, in addition to holograms of distant objects from reference waves.

Introduction

Electron holography was invented by Gabor to correct aberrations of the lenses of an electron microscope. Developments of off-axis electron holography using Möllenstedt-type electron biprisms and a field-emission electron gun [1] expanded application fields of electron holography to observing atomic arrangements [2], [3], magnetic fields [4], [5], electrostatic potentials [6], [7], and strains [8]. An off-axis electron hologram is formed by overlapping an object wave transmitted through a sample with a reference wave passed through a well-known reference area such as a vacuum. The long-standing problem with this method, however, is that the distance between the object and reference waves (D) is limited by lateral coherence length R (which equals,=λ/2β $\propto$ 1/√j, where λ is electron wavelength, β is the half illumination angle of the electron waves, and j is illuminating current density) of the illuminating electron waves [9]. To extend D without decreasing current density of the illuminating electron waves, split-illumination electron holography (SIEH) [10] was developed. SIEH makes it possible to acquire holograms at regions located far from the sample edge or obtain holograms with the reference waves far from the object waves to avoid distortions of the reference waves due to leakage of magnetic or electric fields from the sample.

However, phase shifts of nanoscale electrostatic and magnetic fields are small, it is necessary to improve precise phase measurements of reconstructed object waves from holograms to broaden the applications of the off-axis electron holography. Many technologies, such as phase amplification [11], using reference holograms to remove geometric distortions peculiar to the electron microscope [12], the phase-sifting method [13] and its modifications [14], [15], multiple acquisitions [16], and double-biprism interferometry [17] (enabling independent control of fringe spacing (s) and width (W) of holograms without Fresnel fringes due to Fresnel diffraction at the biprism filament) have been developed. A problem concerning SIEH that remains after these developments was that phase resolution was limited by the modulations of object and reference waves by Fresnel diffraction at the biprism filament in the illuminating system. Given that remaining problem, we have devised an improved version of SIEH, which enables electron holograms to be formed without Fresnel fringes, and applied it to observe electrostatic-potential distributions in semiconductors.

Section snippets

Optical systems

In the case of conventional electron holography, the maximum distance between the object and reference waves (D_max) is limited by the lateral coherence length of the illuminating electron waves (R), and the fringe contrast of the hologram (C) decreases with increasing D (i.e., increasing width (W) of the hologram). Increasing R by decreasing β decreases the current density of the illuminating electron waves. The detectable phase shift in the phase image reconstructed from the hologram is

Methods

The advanced SIEH was realized by installing two biprisms and a third condenser lens in the illuminating system of a cold-field-emission transmission electron microscope (TEM) (HF-3000X, Hitachi High-Technologies Co.). In particular, the double-biprism was installed in the imaging system of the TEM [17]. The TEM was operated at an accelerating voltage of 200 kV. Holograms were acquired using a charge-coupled-device (CCD) camera with 2048×2048 pixels (ORIUS^® SC200, Gatan Inc.) and a slow-scan CCD

Results and discussions

An example of forming interference fringes by the advanced SIEH is shown in Fig. 2. A latex particle (indicated by a green line) is shown as the region of interest on a carbon film, and a vacuum (indicated by orange line) is shown as the reference area [Fig. 2(a)]. First, all the biprism filaments were installed into optical axis and V_CU of −50 V was applied. By increasing V_CL, d_s was increased [Fig. 2(b)]. By applying V_L=150 V to the lower biprism in the imaging system, the same hologram width (W

Conclusion

In conclusion, the remaining problem, namely, phase modulation due to Fresnel fringes, concerning the previously reported SIEH was solved by placing an additional condenser biprism in the illuminating system and focusing the upper condenser-biprism filament onto the sample plane. This advanced SIEH using double biprisms in illuminating system enables flexible control of separation distance d_s and tilting angle θ of the illuminating waves without producing Fresnel fringes onto the sample plane.

Acknowledgments

The authors are grateful to the late Akira Tonomura, who gave us the opportunity to perform this research, for his invaluable discussions. We are also grateful to Kyoichiro Asayama and Naoto Hashikawa of Renesas Electronics Co. for providing nMOS samples and giving their valuable comments as well as to Noboru Moriya of Hitachi Ltd. and Hideki Masuda of Hitachi High-Tech Solutions Co. for their technical support. This research was supported by a grant from the Japan Society for the Promotion of

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View full text

Advanced split-illumination electron holography without Fresnel fringes

Highlights

Abstract

Introduction

Section snippets

Optical systems

Methods

Results and discussions

Conclusion

Acknowledgments

Development of a field emission electron microscope

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Phys. Rev. Lett.

Magnetic microstructure of magnetotactic bacteria by electron holography

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Electron holographic observations of the electrostatic field associated with thin reverse-biased p–n junctions

Phys. Rev. Lett.

Direct observation of potential distribution across Si/Si p–n junctions using off-axis electron holography

Appl. Phys. Lett.

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Nature

Electron Holography

Split-illumination electron holography

Appl. Phys. Lett.

Sensitivity-enhanced electron-holographic interferometry and thickness-measurement applications at atomic scale

Phys. Rev. Lett.

Real-time reconstruction of electron-off-axis holograms recorded with a high pixel CCD camera

J. Comput. Assist. Microsc.